The present disclosure relates to a steam methane reformer (SMR). In particular, the present disclosure relates to a system and method for increasing a carbon monoxide content of syngas produced by SMRs.
Steam methane reformers (SMRs) are generally used as a low-cost option to produce a syngas from a gas feedstock such as natural gas, refinery gas or biogas. The produced syngas can be further processed within the plant to yield various end products, including purified hydrogen, methanol, carbon monoxide and ammonia.
Agriculture operations, decomposition refuse within landfills, municipal water treatment plants, and food and beverage processors generate biomass that must be disposed of in an environmentally friendly and economical manner. Anaerobic digesters can reduce the scale of the biomass by a factor of ten, significantly reducing tipping or disposal fees; however, the digestion process and the decay of the biomass generates methane, or biogas, which may be considered to be an undesirable greenhouse gas. The biogas can be flared, but the flaring process generates pollutants such as nitric oxide (NOx), which creates smog and wastes a potential fuel source. Capturing and using biogas as a fuel to generate liquid fuels solves these challenges in a carbon-neutral manner.
In the steam methane reforming process, high-temperature steam is used to produce syngas from a methane source, such as natural gas or biogas. See FIGS. 1A and 1B. In an endothermic reforming reaction, methane reacts with steam in the presence of a catalyst to produce hydrogen and carbon monoxide, according to the following formula (1):CH4+H2O↔CO+3H2  (1)
At the same time, a slightly exothermic water-gas shift reaction takes place in which the carbon monoxide and steam are reacted using a catalyst to produce carbon dioxide and more hydrogen, according to the following formula (2):CO+H2O↔CO2+H2  (2)The water-gas shift reaction may also be reversed to produce carbon monoxide from carbon dioxide and hydrogen. Syngas, or synthesis gas, is the fuel gas mixture comprised of hydrogen, carbon monoxide, and some carbon dioxide generated from these reactions.
An additional step of pressure-swing adsorption (PSA) may take place in which carbon dioxide and other impurities, such as unconverted methane and water, are removed from the gas stream, leaving essentially only hydrogen and carbon monoxide. This hydrogen and carbon monoxide can then be used to produce higher hydrocarbons such as methanol, other alcohols, or liquids from a Fischer-Tropsch (FT) reaction.
Steam reforming of gaseous hydrocarbons is seen as a potential way to provide hydrogen fuel for low temperature fuel cells. In this case, a lower temperature shift reactor is located between the reformer and the PSA to convert most of the CO from the reformer to H2 and CO2. Also, the CO is removed from the shifted syngas along with the CO2 and other impurities to produce pure H2.
Methanol can be synthesized from syngas according to the following formula (3):CO+2H2↔CH3OH  (3)The reaction of formula (3) may be carried out in the presence of a catalyst, for example, a copper-based catalyst.
Carbon monoxide is preferred over carbon dioxide as a reactant for producing methanol. Therefore, it is desirable to produce syngas with a higher carbon monoxide content than is normally produced from the SMR, especially when methane is the feed gas. Also, a very low carbon dioxide content is desired, since CO2 reacts with H2 to produce CO+H2O, and the H2O reduces the reaction rate and yield of desired products. For best results, the theoretically optimal stoichiometric number is 2.0 for converting syngas to methanol and other FT liquid compounds, as seen in formula (4) below:
                    SN        =                                                            [                                  H                  2                                ]                            -                              [                                  CO                  2                                ]                                                                    [                CO                ]                            +                              [                                  CO                  2                                ]                                              =          2.0                                    (        4        )            In other words, for methanol synthesis, it is desirable to have a 2:1 ratio of hydrogen to carbon monoxide without any CO2.
Dimethyl ether (DME) can be synthesized by methanol dehydration according to the following formula (5):2CH3OH↔CH3OCH3+H2O  (5)
The reaction of formula (5) may be carried out in the presence of a catalyst, for example, a silica-alumina catalyst. The catalyst used in the synthesis of DME is often different from the catalyst used in the synthesis of methanol. The advantage of DME is that it can be used as a direct substitute for diesel fuel in a diesel engine and produces fewer emissions than normal diesel fuel.
In conventional systems in which natural gas is converted to syngas, which is used to produce liquid fuel (see FIG. 1A), the hydrogen to carbon monoxide ratio will be higher (i.e. 6/1 H2/CO) than desired due to the equilibrium composition for the reformed gas. For example, when natural gas from a pipeline is used as the SMR feed, the hydrogen to carbon monoxide ratio produced is much higher than the desired 2/1 ratio (see FIG. 1A). One method to increase CO production in an SMR is to add CO2 to the hydrocarbon feed to the SMR. One way to do this is to use a feed with a high CO2 content. In particular, in order to decrease the hydrogen to carbon monoxide ratio, some systems use biogas/anaerobic digester gas (ADG) as the SMR feed (se FIG. 1B), as opposed to natural gas from a pipeline. For example, the ADG may have a 60/40 methane to carbon dioxide composition. However, the limited biogas availability limits the potential production sites.
A need exists for improved technology, including technology related to a system and method for increasing a carbon monoxide content of syngas produced by steam methane reformers.